Hi,
Yes you can change your board from up board to cbse in class 11th , as registrations for both 11 and 12th class will take place in your 11th class only .
Infact most of the students do so in order to increase their marks as state boards are quite tougher and it is very difficult to score good marks in it. And if you are an above average student , in cbse board you can easily score above 85%. So choose wisely.
Hope it helps....